U.S. patent application number 11/475887 was filed with the patent office on 2007-09-27 for rfid reader/writer.
This patent application is currently assigned to FUJITSU LIMITED. Invention is credited to Toru Maniwa.
Application Number | 20070222608 11/475887 |
Document ID | / |
Family ID | 38042982 |
Filed Date | 2007-09-27 |
United States Patent
Application |
20070222608 |
Kind Code |
A1 |
Maniwa; Toru |
September 27, 2007 |
RFID reader/writer
Abstract
In an RFID reader/writer, a transmitted signal generation unit
generates a carrier wave formed by circular polarization. A first
antenna transmits and receives a polarized wave of a first
polarization component. A second antenna transmits and receives a
polarized wave of a second polarization component orthogonal to the
first polarization component. When a signal analysis unit receives
a reflected wave from a plurality of tags for the carrier waves by
the first and second antennas, the unit analyzes the first and
second polarization components contained in the reflected wave.
Based on the analysis result of the signal analysis unit, electric
power to be transmitted from the first and second antennas is set
to control the polarized wave of a transmission wave to be
transmitted with a command to a plurality of tags.
Inventors: |
Maniwa; Toru; (Kawasaki,
JP) |
Correspondence
Address: |
BINGHAM MCCUTCHEN LLP
2020 K Street, N.W.
Intellectual Property Department
WASHINGTON
DC
20006
US
|
Assignee: |
FUJITSU LIMITED
|
Family ID: |
38042982 |
Appl. No.: |
11/475887 |
Filed: |
June 28, 2006 |
Current U.S.
Class: |
340/572.7 ;
340/10.1; 343/700MS |
Current CPC
Class: |
G06K 7/10316 20130101;
G06K 7/0008 20130101 |
Class at
Publication: |
340/572.7 ;
343/700.0MS; 340/010.1 |
International
Class: |
G08B 13/14 20060101
G08B013/14; H04Q 5/22 20060101 H04Q005/22; H01Q 9/04 20060101
H01Q009/04 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 16, 2006 |
JP |
2006-073539 |
Claims
1. An RFID reader/writer, comprising: a generation unit for
generating a carrier wave of circular polarization; a first antenna
for transmitting a first polarization component of the carrier wave
of circular polarization, and receiving the first polarization
component of a reflected carrier wave; a second antenna for
transmitting a second polarization component orthogonal to the
first polarization component of the carrier wave of circular
polarization, and receiving the second polarization component of
the reflected carrier wave; an analysis unit for analyzing the
first and second polarization components contained in the reflected
wave from a plurality of tags received by the first and second
antennas; and a setting unit for setting transmission power to be
transmitted from the first and second antennas to control the
polarization of the carrier wave to be transmitted with a command
to the plurality of tags based on an analysis result of the
analysis unit.
2. The RFID reader/writer according to claim 1, wherein the first
and second polarization components are constituted by a vertical
component and a horizontal component, and the carrier wave equally
includes a vertical component and a horizontal component.
3. The RFID reader/writer according to claim 2, further comprising
a phase shifter which is arranged between the first and second
antenna and the generation device, and provides a phase shift of
.pi./2 between a vertical component and a horizontal component
contained in the carrier wave.
4. The RFID reader/writer according to claim 2, wherein the
generation unit for comprises a rotation device providing the
second antenna with a .pi./2 phase turned signal from a signal to
be provided for the first antenna.
5. The RFID reader/writer according to claim 1, wherein the first
and second polarization components are respectively a right-handed
polarization component and a left-handed polarization component,
and a carrier wave having only the right-handed polarization
component or the left-handed polarization component is transmitted
through either the first or second antenna.
6. The RFID reader/writer according to claim 1, wherein the setting
unit sets the transmission power from the first and second antennas
using the amplitude of a signal of a polarization component in the
first and second antennas obtained as a result of analysis by the
analysis unit.
7. The RFID reader/writer according to claim 1, wherein the setting
unit sets the transmission power from the first and second antennas
and a phase difference of an electric wave transmitted from each of
the antennas using amplitude and a phase of a signal of a
polarization component in the first and second antennas obtained as
a result of analysis by the analysis unit.
8. An RFID reader/writer, comprising: a generation unit for
generating a carrier wave of linear polarization; a first antenna
for use in transmitting a first polarization component of a carrier
wave of the linear polarization, and receiving a first polarization
component of a reflected carrier wave; a second antenna for use in
transmitting a second polarization component orthogonal to the
first polarization component in a carrier wave of the linear
polarization, and receiving a second polarization component of a
reflected carrier wave; an analysis unit for analyzing the first
and second polarization components included in reflected waves from
a plurality of tags received by the first and second antennas; and
a setting unit for setting transmission power to be transmitted
from the first and second antennas to control the polarization of a
carrier wave to be transmitted with a command to the plurality of
tags based on an analysis result of the analysis device.
9. A method for transmitting a command to a plurality of tags from
an RFID reader/writer, comprising the processes of: generating a
carrier wave of circular polarization; analyzing a first
polarization component and a second polarization component
contained in reflected waves from a plurality of tags received by a
first antenna transmitting the first polarization component of the
carrier wave of circular polarization, and receiving the first
polarization component of a reflected carrier wave, and a second
antenna transmitting the second polarization component orthogonal
to the first polarization component of the carrier wave of circular
polarization, and receiving the second polarization component of a
reflected carrier wave; and setting transmission power to be
transmitted from the first and second antennas to control
polarization of the carrier wave to be transmitted with a command
to the plurality of tags based on an analysis result of the
analysis device.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a reader/writer for use in
a Radio Frequency Identification (RFID) system.
[0003] 2. Description of the Related Art
[0004] The conventional method of enhancing the communication
performance between a RFID reader/writer and a RFID tag, when
reading a plurality of RFID tags, is by optimally switching
polarity after reading each tag. In this method, the polarization
is switched for each tag, which in turn causes large delays in
switching antennas, further causing a failure to read information
at a high speed.
[0005] Furthermore, a technology enabling collective reading
(anti-collision) of RFID also exists, which enables the
simultaneous reading of a plurality of tags. To steadily read data
independent of the direction of a tag, in many cases a circularly
polarized antenna is used as the antenna for the RFID tag
reader/writer.
[0006] Alternatively, an antenna of a common tag is linear and
receives only a linear polarization component. Since the direction
of a tag is uncertain during use, the direction of an antenna is
also uncertain. When there are a number of tags, it is possible
that power cannot be evenly supplied to all tags, creating a
situation where there can be a tag not added to the anti-collision
protocol, and there fore the information from all tags cannot be
read.
[0007] Relating to the technology of wirelessly collecting data in
equipment, there are techniques of using an antenna for oval
polarization, circular polarization, or linear polarization by
changing the phase between power supply points of patch antennas
(for example, refer to patent document 1). [0008] [Patent Document
1] Japanese Patent Application Laid-open No. H10-260251
[0009] As noted above, it is not possible to realize high speed
reading of the RFID tag when switching polarization for each of the
tags. If a simultaneous read is attempted from a plurality of tags
by anti-collision when the tags are arranged at random, then the
direction of the antenna is also set at random, and there is the
possibility that power cannot be evenly supplied to all tags
although circular polarization is transmitted from the RFID
reader/writer.
SUMMARY OF THE INVENTION
[0010] The present invention aims at providing the technology of
supplying power to a plurality of tags at high speed and
efficiently.
[0011] To avoid the above-mentioned problems, the RFID
reader/writer according to the present invention includes: a
generation unit for generating a carrier wave of circular
polarization; a first antenna for transmitting a first polarization
component of the carrier wave of circular polarization, and
receiving the first polarization component of a reflected carrier
wave; a second antenna for transmitting a second polarization
component orthogonal to the first polarization component of the
carrier wave of circular polarization, and receiving the second
polarization component of the reflected carrier wave; an analysis
unit for analyzing the first and second polarization components
contained in the reflected wave from a plurality of tags received
by the first and second antennas; and a setting unit for setting
transmission power to be transmitted from the first and second
antennas to control the polarization of the carrier wave to be
transmitted with a command to the plurality of tags based on an
analysis result of the analysis unit.
[0012] Before transmitting a command, the carrier wave of circular
polarization is transmitted. The first polarization component of
the carrier wave is transmitted from the first antenna, and the
second polarization component is transmitted from the second
antenna. In the reflected waved emitted from the plurality of tags
using a linearly polarized antenna, the first antenna receives the
first polarization component, and the second antenna receives the
second polarization component. By analyzing the signal received by
each antenna, and in order to control the polarization of the
carrier wave to be transmitted by a command based on the analysis
result, the transmission power for each antenna is set. Thus, the
power can be set corresponding to the polarization distribution of
the plurality of tags.
[0013] The first and second polarization components are, for
example, a vertical component and a horizontal component, or a
right-handed polarization component and a left-handed polarization
component. Depending on the configuration of the receiver, the
phase shift of .pi./2 can be set between the first and second
polarization components. In addition, the power for each
polarization component may also be configured by the use, with the
power setting to be transmitted as a command.
[0014] Alternatively, the RFID reader/writer according to the
present invention includes: a generation unit for generating a
carrier wave of linear polarization; a first antenna for
transmitting a first polarization component of a carrier wave of
the linear polarization, and receiving a first polarization
component of a reflected carrier wave; a second antenna for
transmitting a second polarization component orthogonal to the
first polarization component of the carrier wave of linear
polarization, and receiving a second polarization component of a
reflected carrier wave; an analysis unit for analyzing the first
and second polarization components contained in the reflected wave
from a plurality of tags received by the first and second antennas;
and a setting unit for setting transmission power to be transmitted
from the first and second antennas to control the polarization of
the carrier wave to be transmitted with a command to the plurality
of tags based on an analysis result of the analysis unit. When a
circularly polarized antenna is used as the antenna for a RFID tag,
a transmission power command corresponding to the distribution of
the polarization of a plurality of tags can be set as described
above.
[0015] According to the present invention, electric power is more
efficiently provided for a plurality of RFID tags. Therefore, the
number of switches of the polarization to read information from the
plurality of tags can be reduced, and the reading process can be
performed at a higher speed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 shows the configuration of the RFID reader/writer
according to a style of embodiment of the present embodiment,
[0017] FIG. 2A is an explanatory view (1) of the basic operation of
the reader/writer,
[0018] FIG. 2B is an explanatory view (2) of the basic operation of
the reader/writer,
[0019] FIG. 2C is an explanatory view (3) of the basic operation of
the reader/writer,
[0020] FIG. 3 shows the configuration of the reader/writer having a
separate transmission/reception system for the vertical component
and the horizontal component,
[0021] FIG. 4A is an explanatory view (1) of a series of operations
from determining the vertical component and the horizontal
component to transmitting a command,
[0022] FIG. 4B is an explanatory view (2) of a series of operations
from determining the vertical component and the horizontal
component to transmitting a command,
[0023] FIG. 4C is an explanatory view (3) of a series of operations
from determining the vertical component and the horizontal
component to transmitting a command,
[0024] FIG. 5 shows the configuration of the reader/writer having a
separate transmission/reception system for the right-hand
polarization component and the left-hand polarization
component,
[0025] FIG. 6A is an explanatory view (1) of a series of operations
from determining the right-hand polarization component and the
left-hand polarization component to transmitting a command,
[0026] FIG. 6B is an explanatory view (2) of a series of operations
from determining the right-hand polarization component and the
left-hand polarization component to transmitting a command,
[0027] FIG. 6C is an explanatory view (3) of a series of operations
from determining the right-hand polarization component and the
left-hand polarization component to transmitting a command,
[0028] FIG. 7 is a flowchart of the process of determining the
power ratio between the polarization components of the commands
depending on the distribution of tags,
[0029] FIG. 8 shows the configuration of the reader/writer provided
with a separate transmission/reception system for the vertical
component and the horizontal component,
[0030] FIG. 9A is an explanatory view (1) of a series of operations
from determining the vertical component and the horizontal
component to transmitting a command,
[0031] FIG. 9B is an explanatory view (2) of a series of operations
from determining the vertical component and the horizontal
component to transmitting a command,
[0032] FIG. 9C is an explanatory view (3) of a series of operations
from determining the vertical component and the horizontal
component to transmitting a command,
[0033] FIG. 10 shows another configuration of the reader/writer
provided with a separate transmission/reception system for the
vertical component and the horizontal component,
[0034] FIG. 11A is an explanatory view (1) of the process of
determining the right-hand polarization component and the left-hand
polarization component and transmitting a command,
[0035] FIG. 11B is an explanatory view (2) of the process of
determining the right-hand polarization component and the left-hand
polarization component and transmitting a command,
[0036] FIG. 11C is an explanatory view (3) of the process of
determining the right-hand polarization component and the left-hand
polarization component and transmitting a command, and
[0037] FIG. 12 is a flowchart of the process of determining the
power ratio and the phase difference between the polarization
components of the commands depending on the distribution of
tags.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0038] It is preferable that the polarized wave transmitted from a
reader/writer to a RFID tag is an oval polarization depending on
the distribution of a plurality of tags. Through the usage of oval
polarization for transmission to the tags from the reader/writer,
power can be supplied efficiently to a plurality of tags even when
anti-collision is performed. Therefore, in the manner of embodiment
of the present invention, the reader/writer transmits a
transmission wave of appropriate oval polarization.
[0039] Preferable manner of embodiments according to the present
invention are described below in detail by referring to the
attached drawings.
[0040] FIG. 1 (FIG. 1) shows the configuration of the RFID
reader/writer according to a manner of embodiment of the present
invention. An RFID reader/writer (hereinafter referred to as a
reader/writer for short) 50 is provided with two
transmission/reception systems, and comprises a signal-processing
unit 10, an orthogonal modulation unit 3 (3A, 3B), a transmitter 4
(4A, 4B), a separator 5 (5A, 5B), an antenna 6 (6A, 6B), areceiver
7 (7A, 7B), and an orthogonal demodulation unit 8 (8A, 8B). The
reader/writer 50 communicates with a plurality of tags T1, T2, . .
. , TN without contact, reads the information about the tags, and
writes information to the tags. In FIG. 1, only the n-th tag is
described without describing a plurality of tags. In the following
explanation, when it is not necessary to designate which system in
the two transmission/reception systems is, the characters A and B
are omitted.
[0041] The signal-processing unit 10 includes a DSP (digital signal
processor), among other components, for processing a digital signal
obtained from the transmission wave transmitted from the
reader/writer 50 to the tag and the reflected wave from the tag.
The digital signal generated in the signal-processing unit 10 is
carried on the carrier wave in the orthogonal modulation unit 3,
and externally transmitted through the transmitter 4 and the
antenna 6. When the RFID tag receives an electromagnetic wave from
the reader/writer, it reflects a wave back to the reader/writer.
When antenna 6 receives the reflected wave, it is transmitted to
the orthogonal demodulation unit 8 through the receiver 7, and a
digital signal is retrieved. The retrieved signal is then supplied
to the signal-processing unit 10, and various processes are
performed. The separator 5 separates the transmission wave and the
received wave of each polarization component.
[0042] The two transmission/reception systems transmit and receive
the polarization components orthogonally to each other. For
example, one transmission/reception system is provided for
transmitting/receiving a vertical component, and the other for
transmitting/receiving a horizontal component. Alternatively, one
transmission/reception system is provided for
transmitting/receiving a right-handed polarization component, and
the other transmission/reception system for transmitting/receiving
a left-handed polarization component. In the following explanation,
the two systems respectively are referred to as system A and system
B.
[0043] The configuration of the signal-processing unit 10 includes
a transmission signal generation unit 1, a (complex) arithmetic
unit 2, and a received signal analysis unit 9 (9A, 9B). The
transmission signal generation unit 1 generates a signal for
transmission of an electromagnetic wave to a tag as described
above. The (complex) arithmetic unit 2 calculates, based on the
power Pa for the polarized waves transmitted from system A, the
power Pb for the polarization component (orthogonal to the system
A) transmitted from system B, and the phase difference between the
polarization components. The received signal analysis unit 9
analyzes the signal provided from the orthogonal demodulation unit
8 of each system. The (complex) arithmetic unit 2 only calculates
the scalar amount without performing a complex arithmetic operation
depending on the configuration of the transmission/reception system
of reader/writer 50.
[0044] The following example describes the case in which the
reflected wave transmitted from the RFID tag T to the carrier wave
transmitted from the reader/writer 50 though linear
polarization.
[0045] FIGS. 2A, 2B, and 2C are explanatory views of the basic
operations of the reader/writer 50 as shown in FIG. 1. As shown in
FIG. 2A, the reader/writer 50 first transmits continuous carrier
waves (CW, constant wave) to a plurality of tags T. In this
example, circular polarization as a non-modulated wave is
transmitted. Following that, as shown in FIG. 2, the reader/writer
50 receives reflected waves from a plurality of tags (T1, T2, . . .
, TN) in response to the carrier waves. The antennas 6A and 6B,
respectively, receive the reflected waves of a polarization
component corresponding to the polarization component of the
transmitted carrier waves. A received baseband signal (hereinafter
referred to simply as a signal for short) is analyzed by the
received signal analysis unit 9 of the signal processing unit 10,
which determines the value of the signal amplitude: a, b, and so
on. Finally, as shown in FIG. 2C, based on the analysis result as
shown in FIG. 2B, the power transmitted from systems A and B of the
reader/writer 50, that is, the power for each polarization
component, is determined, and transmits a command to a tag.
Hereinafter, setting the power, for example, of a command
transmitted from the transmission/reception systems A and B to a
tag is expressed as "weighting".
[0046] As described above in the explanation shown by referring to
FIG. 1, the polarization components received by the two
transmission/reception systems are orthogonal to each other, and
the vertical component and the horizontal component or the
right-handed polarization component and the left-handed
polarization component are shown as examples. Described below are
the configuration and the operation of the reader/writer depending
on the type of polarized antenna.
[0047] FIG. 3 shows the configuration of the reader/writer 50
provided with the transmission/reception systems for the vertical
component and the horizontal component. In the configuration shown
in FIG. 3, a configuration diagram similar to that shown in FIG. 1
is omitted here.
[0048] Two antennas 6 can be dipole antennas. FIG. 3 shows the
antenna 6A for transmitting/receiving the polarized waves of the
vertical polarization component and the antenna 6B for
transmitting/receiving the polarized waves of the horizontal
polarization component.
[0049] A ninety-degree (90.degree.) phase shifter 11 is provided
immediately below the two antennas 6A and 6B. By passing the
polarized waves transmitted from the antennas 6A and 6B through the
90.degree. phase shifter 11, the phase difference between the
vertical component and the horizontal component of the polarized
waves transmitted from the reader/writer 50 is assumed to be
90.degree.. Thus, the electromagnetic wave transmitted from the
antenna 6 to the tag becomes a circular polarization equally
containing the vertical component and the horizontal component.
Related to the transmission/reception systems, the orthogonal
demodulation unit 8 is provided in FIG. 1, but a detector 13 is
provided in the configuration shown in FIG. 3.
[0050] FIGS. 4A, 4B, and 4C are explanatory views of a series of
operations from weighting the vertical component and the horizontal
component to transmitting a command. In FIGS. 4A, 4B, and 4C, of
the two orthogonal polarized antennas, the antenna for the vertical
polarization component corresponds to the antenna 6A shown in FIG.
1, and the antenna for the horizontal polarization component
corresponds to the antenna 6B shown in FIG. 1.
[0051] First, as shown in FIG. 4A, the reader/writer 50 transmits a
carrier wave (CW). As described above, the two dipole antennas 6A
and 6B, respectively, transmit the polarized waves of the vertical
component and the horizontal component. Since the strength of each
component is equal, and the phase difference is set to 90.degree.,
the carrier wave is transmitted with circular polarization.
[0052] As shown in FIG. 4B, the signal of the vertical component
and the horizontal component detected by the vertical component
monitor (detector) 13A and the horizontal component monitor
(detector) 13B of the reader/writer 50 are analyzed by their
respective received signal analysis unit 9, and the respectively
received components a and b are obtained. In this example, a and b
indicate the scalar amount. Since the tag is provided with a
linearly polarized antenna, it returns to the reader/writer 50 a
reflected wave for the polarization component corresponding to the
direction of each tag (antenna) in the polarized waves of the
vertical component and the horizontal component transmitted from
the reader/writer 50. That is, the reader/writer 50 receives a
reflected wave composed of a ratio of the strength of the vertical
component and the strength of the horizontal component based on the
distribution of the directions of the antennas of a plurality of
tags. In the antennas of a plurality of tags, for example, if a
number of antennas are arranged in the horizontal polarization
component direction, the horizontal polarization component of the
reflected wave is larger than the vertical polarization component
relatively. If the antennas of all tags are arranged in the
horizontal component direction, only the horizontal component is
detected in the reflected wave.
[0053] Based on the obtained received components a and b, the power
(Pa and Pb) for the command to be transmitted from the
reader/writer 50 to the tag is calculated. Assuming that the power
to be transmitted from the reader/writer 50 is P, the power
transmitted from the antennas 6A and 6B is obtained by the
following equations: Pa={a/(a.sup.2+b.sup.2)}.times.P (1)
Pb={b/(a.sup.2+b.sup.2)}.times.P (2)
[0054] As shown in the preceding equations, the ratio of the
transmission strength of the vertical polarization component to the
transmission strength of the horizontal polarization component is
set to a to b. That is a command of oval polarization satisfying
the equations (1) and (2) is transmitted.
[0055] FIG. 5 shows the configuration of the reader/writer 50
provided with a transmission/reception system for the right-handed
polarization component and the left-handed polarization component.
In this example, in the configuration shown in FIG. 5, the
configuration differs from that of the reader/writer shown in FIG.
3 due to the antennas 6A and 6B. Each antenna of the right-handed
polarization antenna 6A and the left-handed polarization antenna 6B
transmits the polarized waves of the right-handed polarization
component and the left-handed polarization component
correspondingly.
[0056] FIGS. 6A, 6B, and 6C are explanatory views of a series of
operations from weighting the right-handed polarization component
and the left-handed polarization component to transmitting a
command. Described below are the differences from the situation in
which the vertical component and the horizontal component are
weighted using two dipole antennas as shown in FIGS. 4A, 4B, and
4C.
[0057] First, as shown in FIG. 6A, the carrier wave transmitted
from the reader/writer 50 is a circular polarization as described
above. In the two polarized antennas, the circular polarization is
transmitted from one antenna, that is, from the right-handed
polarization antenna in the example shown in FIG. 6A.
[0058] Then, as shown in FIG. 6B, the reflected wave from the
carrier wave from each tag is received by the reader/writer 50 and
each component is detected, and the signal analysis units 9A and 9B
analyze the value of each component. The values of the obtained
right-handed polarization component and left-handed polarization
component are respectively set as a and b (a and b indicate scalar
amounts). The method of obtaining the values of a and b depending
on the arrangement of the antennas of a plurality of tags is
described above. Finally, as shown in FIG. 6C, based on the
obtained values a and b, the power Pa and Pb of the command
transmitted respectively from the antennas 6A and 6B is obtained.
The equations for obtaining the Pa and Pb are identical to those
listed above, the explanation is omitted here.
[0059] FIG. 7 is a flowchart of the process for determining the
power for each polarization component of the command depending on
the distribution of tags. The series of processes shown in FIG. 7
are performed with the timing of, for example, starting the
anti-collision (collective read) process by the reader/writer 50 or
the timing of starting the process of transmitting a command to a
tag during the execution of the anti-collision process.
[0060] First, in step S1, the polarized wave, (carrier wave, CW)
used as a reference for checking the polarization direction of a
tag, is transmitted. The transmitted carrier wave has circular
polarization as described above. Then, in step S2, the carrier
waves emitted again (reflected) by the tags are received by the
reader/writer 50, and the values of the polarization components
orthogonal to each other are obtained. The polarization components
a and b obtained by the reader/writer 50, provided with the
configuration shown in FIGS. 3 and 5, are a scalar amount. Finally,
in step S3, the above-mentioned Pa and Pb are set as the
transmission power of each polarization component when the
transmission power is set as P, and a command provided with the set
power is transmitted, thereby terminating the process.
[0061] As explained above by referring to FIGS. 3 through 7,
according to the reader/writer 50 of this manner of embodiment of
the present invention, the circular polarization is transmitted as
a non-modulated wave before transmitting a command to a plurality
of tags, and the detector 13 detects a received signal for each of
the two polarization components orthogonal to each other relating
to the reflected waves from the tags. Based on the value of the
received signal for each of the obtained polarization components,
the power to be transmitted for each polarization component is
determined. The polarization component of the reflected wave from a
plurality of tags, for determination of power for each polarization
component, is determined depending on the positional relationship
between the reader/writer 50 and the plurality of tags T.
Therefore, although a plurality of tags as the destinations of a
command are arranged at random, a command can be transmitted with
the oval polarization depending on the arrangement of tags. Thus,
the frequency of switching the polarization can be reduced in the
process such as the anti-collision, for example, and the time
required to process the anti-collision, among other things, can be
shortened.
[0062] The reader/writer according to the manner of embodiment
described above determines the size of the transmission power of a
command for each polarization component. Described below is a
reader/writer capable of setting the phase difference between the
polarization components in addition to the power of a command.
[0063] FIG. 8 shows the configuration of the reader/writer 50
provided with a separate transmission/reception system for the
vertical component and the horizontal component. When the
reader/writer shown in FIG. 8 is compared with the reader/writer
shown in FIG. 3, the means for detecting the received signal
contained in the reflected wave corresponds to the detector 13 in
FIG. 3 while the orthogonal demodulation unit 8 corresponds to it
in FIG. 8. In FIG. 8, the configuration of the signal processing
unit 10 is omitted, but it is similar to that shown in FIG. 1.
[0064] The demodulation unit 8 is formed by an orthogonal
demodulation circuit, among other components, and can detect the
amplitude and the phase of each polarization component of the
signal received by an antenna. The amplitude and the phase of an
obtained signal are provided to the signal-processing unit 10. The
signal-processing unit 10 calculates the power and the phase
difference between the two polarized waves for each polarization
component of a command based on the amplitude and phase difference
of each polarization component.
[0065] FIGS. 9A, 9B, and 9C are explanatory views of a series of
operations from weighting the vertical component and the horizontal
component to transmitting a command in the reader/writer shown in
FIG. 8. Described are the differences in the operations for this
manner of embodiment with the operations of the reader/writer
provided with the configuration shown in FIG. 3.
[0066] The transmission of the carrier wave shown in FIG. 9A, as in
FIG. 4A, transmits the circular polarization which includes equally
the vertical component and the horizontal component. As shown in
FIG. 9B, when the reflected waves are received by the reader/writer
50 from a plurality of tags, the signals of the vertical component
and the horizontal component are respectively provided for the
vertical component monitor (corresponding to the demodulation unit
8A shown in FIG. 8) and the horizontal component monitor
(corresponding to the demodulation unit 8B shown in FIG. 8) and
analyzed. The value indicating each polarization component acquired
as a result of the analysis is defined as a and b. In this example,
a and b indicates the amount of the vectors, a complex number.
[0067] Based on the received components a and b, the power (Pa and
Pb) of the command transmitted from the reader/writer 50 to the
tags and the phase difference between the polarized waves
transmitted from the two antennas 6A and 6B are set. The power
transmitted from each of the antennas 6A and 6B with the
transmission power set as P can be expressed by the following
equations (3) and (4); the phase difference between the polarized
waves transmitted from the two antennas 6A and 6B is given by
(.angle.a-.angle.b): Pa={|a|/(|a|.sup.2+|b|.sup.2)}.times.P (3)
Pb={|b|/(|a|.sup.2+|b|.sup.2)}.times.P (4)
[0068] FIG. 10 shows another configuration of the reader/writer 50
provided with a separate transmission/reception system for the
vertical component and the horizontal component. As compared with
the configuration shown in FIG. 8, the 90.degree. phase shifter 11
is not provided immediately below the two antennas, and a rotation
unit 12 is provided inside the signal processing unit 10.
[0069] The rotation unit 12 turns the phase of the signal
transmitted to the transmission/reception system A 90.degree., and
provides it for the transmission/reception system B. Thus, the
polarized wave transmitted from the antenna 6A is orthogonal to the
polarized wave transmitted from the antenna 6B. The reader/writer
50 having the configuration shown in FIG. 10, using two dipole
antennas, transmits from the antennas 6A and 6B the polarized waves
of the vertical component and the horizontal component. With the
reader/writer 50 having the above-mentioned configuration, the
power Pa and Pb of the command to be transmitted to the tag and the
phase difference (.angle.a-.angle.b) can be set.
[0070] FIGS. 11A, 11B, and 11C are explanatory views of the process
of transmitting a command after weighting it for the right-handed
polarization component and the left-handed polarization component.
Related to the configuration of the reader/writer 50, a signal is
detected by the detector 13 in FIG. 5 while an orthogonal
demodulation circuit, for example, detects a signal and a phase.
Otherwise, the configurations are the same as those shown in FIG.
5. In this example, the differences from the operations of the
reader/writer shown in FIGS. 6A, 6B, and 6C are described.
[0071] In the process shown in FIG. 11A, for the circularly
polarized waves transmitted from one (in FIGS. 11A, 11B, and 11C,
the antenna 6A for right-handed polarization) of the two antennas,
the reflected waves are received by the reader/writer 50 as shown
in FIG. 11B. The antenna 6A receives the right-handed polarization
component, and the antenna 6B receives the left-handed polarization
component. The signal of each polarization component included in
the reflected waves received by the antenna is provided for the
right-handed component monitor (orthogonal demodulation circuit) 8A
and the left-handed component monitor (orthogonal demodulation
circuit) 8B, and the amplitude and the phase of the signal are
analyzed. The values obtained as a result of the analysis are set
as a and b (a and b are vector amounts). The method for obtaining
the power and the phase difference for each polarization component
in FIG. 11C is similar to that shown in FIG. 9C. Therefore, the
explanation is omitted here.
[0072] FIG. 12 is a flowchart of the process of determining the
power ratio and the phase difference between the polarization
components of the commands depending on the distribution of tags. A
series of processes shown in FIG. 12 are performed with the timing
of starting the anti-collision process or with the timing of
starting the transmission of a command for the tags during the
execution of the anti-collision process.
[0073] Since the process in step S11 is similar to the process in
step S1 shown in FIG. 7, the explanation is omitted here. In step
S12, for the signal emitted from the tag, the components a and b
are obtained from each polarization component. As described above,
a and b indicate a vector amount (which is a complex number). In
step S13, using the amounts a and b, the power for each
polarization component about the command to be transmitted and the
phase difference (.angle.a-.angle.b) between the polarization
component are determined, and the command having the power and
phase difference is transmitted, subsequently terminating the
process.
[0074] The reader/writer 50 explained by referring to FIG. 8
through FIGS. 11A, 11B, and 11C sets the power for each
polarization component and the phase difference between the
polarization components. Therefore, the command transmitted by the
reader/writer 50 indicates an oval having an inclination. As
compared with the reader/writer 50 shown from FIG. 3 to FIG. 7, a
command corresponding to the arrangement of tags can be
transmitted.
[0075] In the embodiments above, solely explained is the case in
which the antenna of a tag is a linearly polarized antenna.
However, the present invention is not limited to this application.
As described in the embodiment above, when the antenna of a tag is
a linearly polarized antenna, two systems exist, each transmitting
the polarized waves of the vertical component and the horizontal
component or the right-handed polarization component and the
left-handed polarization component, and the carrier wave
transmitted from the reader/writer 50 to analyze the polarization
component depending on the distribution of tags is a circularly
polarized wave. Additionally, for example, when a circularly
polarized antenna constitutes the antenna of a tag, an antenna of a
reader/writer can transmit the linearly polarized waves constituted
by including equally the right-handed polarization component and
the left-handed polarization component as carrier waves. Otherwise,
when the two antennas transmit and receive the vertical component
and the horizontal component, the carrier wave can be transmitted
from one antenna. With the reader/writer having the above-mentioned
configuration, depending on the distribution of the polarized waves
of a plurality of tags having a circularly polarized antenna, the
reflected wave can be received, thereby enabling the appropriate
power setting for each polarization component of the command.
* * * * *